I. Field of the Invention
The present invention relates generally to ophthalmic instruments, and more particularly to a portable hand-held non-contact tonometer for measuring intraocular pressure of a patient""s eye.
II. Description of the Related Art
Non-contact tonometers are diagnostic instruments widely used by ophthalmologists and medical personnel for measuring the internal fluid pressure within the eye (intraocular pressure or IOP), often to screen patients for elevated IOP associated with glaucoma. Non-contact tonometers typically operate by directing a fluid pulse at the eye and observing deformation of the cornea. In conventional apparatus of the prior art, a fluid pump having a solenoid-driven piston compresses fluid within a plenum chamber, and a fluid discharge tube in communication with the plenum chamber and aligned with the patient""s eye delivers a fluid pulse to the eye that deforms the cornea from its normal convex state, through a flattened state known as xe2x80x9capplanation,xe2x80x9d to a concave state. When the fluid pulse dissipates, the cornea returns to its normal convex state. The deformation is monitored by opto-electronic means, and a quantity such as the plenum pressure at the moment of applanation or the time required to achieve applanation is measured and correlated to IOP.
Heretofore, non-contact tonometers have been primarily bulky xe2x80x9ctable topxe2x80x9d instruments that are not easily portable. In practice, the patient sits at one end of the instrument with his or her head steadied by a forehead brace, and the operator sits at the opposite end to align the instrument relative to the eye and administer the test. The instrument, which contains precisely aligned optical components, remains stationary on the table except for a test portion that moves relative to a base of the instrument for alignment purposes.
The desirability of a smaller, lightweight instrument for measuring IOP has been recognized for some time, as evidenced by the development of hand-held xe2x80x9ccontactxe2x80x9d type tonometers. See for example, U.S. Pat. Nos. 4,192,317; 4,622,459; 4,747,296; and 5,174,292. Because a portion of the tonometer physically contacts the cornea, these instruments are generally regarded as being less comfortable to the patient than the non-contact variety described above, and there is an increased risk of infection because viruses and bacteria can be transferred from one patient to the next. Moreover, an operator""s skill in testing can have a significant impact upon measurement results, thus rendering these instruments poorly suited for use by general medical practitioners.
U.S. Pat. No. 4,724,843 describes a portable non-contact tonometer that includes a carrying case 102 for housing a pump used to generate a fluid pulse, and a detachable hand-held unit 100 connected to the pump by a flexible connection line 104 enclosing a fluid conduit. Thus, only a portion of the instrument is hand-held, with the remainder of the instrument being large and heavy. The non-contact tonometer described in U.S. Pat. No. 4,724,843 is complex and expensive to manufacture.
Therefore, it is an object of the present invention to provide a non-contact tonometer that is designed for hand-held use.
It is another object of the present invention to provide a non-contact tonometer that is compact and lightweight for easy transport.
It is yet another object of the present invention to provide a non-contact tonometer that is battery powered to avoid the use of power cords.
It is yet another object of the present invention to provide a compact handheld non-contact tonometer that is designed so as to reduce the risk of damage to internal components thereof.
It is yet another object of the present invention to provide a hand-held non-contact tonometer having a fast position detection system for facilitating hand-operated alignment of the tonometer.
It is yet another object of the present invention to provide a non-contact tonometer that is equipped for wireless data communication with a remote computer to enable uploading of measurement data without need for communication cables.
In furtherance of these and other objects, a non-contact tonometer according to a preferred embodiment of the present invention includes a housing having a handle portion that encloses a rechargeable D.C. power source and an upper head portion that encloses alignment and tonometric measurement systems of the tonometer. The bottom of the handle portion defines a curved surface that prevents the instrument from being rested on the handle alone, because this would be a precarious state given the tonometer""s high center of gravity and the fact that critical optical and opto-electronic elements are located within the head portion of the housing. A stand comprising a battery recharger is provided in combination with the tonometer for supporting the tonometer and recharging the tonometer""s power source while the instrument is not in use.
An operator must position and align the tonometer by hand relative to a patient""s eye. To assist the operator, an optical axis extends through the head portion from the patient end where a fluid discharge tube is located to the operator end where an eyepiece is located, and the operator is afforded a direct view of the eye without the use of a camera or the like. A fast afocal position detection system is also incorporated in the head portion for determining X-Y-Z alignment status of the tonometer relative to the patient""s eye. The position detection system comprises first and second light sources on opposite sides of the optical axis, and corresponding first and second light-sensitive area detectors positioned to receive light from an associated light source after it has been reflected by the cornea. The detectors provide signal information indicative of the local x-y position of an illumination spot formed thereon. In the preferred embodiment, the first and second detectors are quad-cell detectors having four quadrants, and the illumination spot size is about the size of one quadrant, whereby the x-y position can be determined based on the four signal levels generated by the quadrants. Collector lenses after each light source and in front of each detector minimize vergence in the light beam as it illuminates the eye and as it arrives at a detector.
The local x-y data from each detector are then provided as input to a series of stored geometrical relationships determined during instrument calibration for giving the X-Y-Z global alignment status of the tonometer relative to the eye. The geometrical relationships are multiple regression equations for X, Y, and Z, wherein regression coefficients for each equation are determined by reading local x-y data from the detectors for an artificial eye placed at a plurality of known X-Y-Z positions during calibration. The regression coefficients are stored and used during normal instrument operation to quickly calculate X, Y and Z coordinates based on local x-y data from the detectors as an operator positions the instrument relative to a patient""s eye.
A xe2x80x9cheads-upxe2x80x9d display is connected to receive the X-Y-Z position data and provide instructional cues to the operator for moving the instrument to achieve alignment. The heads-up display comprises a polar array of light emitting diodes selectively illuminated to indicate a desired X-Y movement direction, and a linear array of light emitting diodes selectively illuminated to indicate a desired Z movement direction. An image of the heads-up display is presented to the operator along the instrument optical axis through the use of a beamsplitter that allows the directly viewed image of the patient""s eye to be transmitted as well along the optical axis, whereby the X-Y polar array is arranged circumferentially about the directly viewed image.
Tonometric measurement is carried out by generating a fluid pulse to deform the cornea, observing the occurrence of corneal applanation, and correlating a plenum pressure associated with the fluid pulse at the time of applanation with IOP. The tonometer thus includes a solenoid that is energized when the position detection system confirms alignment, a piston driven by the solenoid relative to a cylinder to generate a fluid pulse directed through the fluid discharge tube toward the patient""s eye, a pressure sensor for monitoring plenum pressure, and an applanation emitter and applanation detector on opposite sides of the optical axis for observing applanation based on corneally reflected light.
The tonometer further comprises an IRDA infrared data association (IRDA) transceiver for wireless uploading of measurement data to a remote computer.